(1) Field of the Invention
The present invention relates to a method for locating positions of each wireless terminal with use of signals received from a plurality of radio wave transmitting sources. More particularly, the present invention relates to a method used by a system for locating a position of each of its wireless terminals with use of signals received from a plurality of cellular base stations.
(2) Description of Related Art
FIG. 12 shows a block diagram of a wireless terminal that employs a conventional technique. The principle of a trilateration method for detecting a position of such a wireless terminal with use of signals received from a plurality of wave transmitting sources according to the conventional technique is disclosed in the patent document 1.
At first, the configuration of the wireless terminal shown in FIG. 12 will be described with reference to the IS-95. The terminal comprises an antenna 1; an RF unit 2; a base band unit 3; a memory 7; and a CPU 8. Signals received by the antenna 1 are converted to base band signals by the RF unit 2.
Next, a procedure for locating a position of a terminal will be described. Each IS-95 base station transmits its pilot signal having a fixed pattern. The base station transmits each signal at a timing decided by its own unique PN offset so as to be delayed from the system clock. The terminal actuates its pilot signal correlator 6 to decide the nearest base station. The terminal then refers to all the phases of the pilot signal of the correlator 6 sequentially, to be correlated and find a timing at which the correlation peak is detected. The detected maximum correlation peak timing denotes a timing for synchronizing the terminal itself with the possibly nearest base station. The base band unit 3 includes a despreader 4 for control channel in itself and the despreader 4 performs a despreading operation at the timing of the detected nearest base station to despread a control channel signal from received base band signal. The despread control channel signal is detected by the receiver 5 and demodulated to valuable information therein. The CPU 8 takes out the ID of the subject base station from the detected information, then refers to the information tables of the near base stations to be observed, stored in the memory 7 beforehand to take out the PN offset of each of the near base stations. The terminal then generates complex delay profiles using the pilot signal correlator 6 with respect to the timing of each of the nearest and near base stations. Furthermore, the terminal converts the complex information to power delay profiles through sum-of-product operations. The generated power delay profiles are stored in the memory 7. The CPU 8 analyzes each delay profile stored in the memory 7 to take out a path-detected timing. The CPU 8 then locates the position of the terminal using such a solution method as the least squares method.
To receive signals from a plurality of base stations simultaneously, the terminal may be provided with a plurality of the pilot signal correlators 6. It is also possible to store received signals (snapshots) in the memory 7 once, then call the snapshots from the memory 7 sequentially by changing the PN offset in the correlator 6 and input them in the correlator 6. By repeating the procedure for generating delay profiles from the snapshots stored in this memory 7, delay profiles corresponding to a plurality of base stations can be generated from the same snapshot.
To receive signals at an apparatus for wireless position location effectively, for example, inside a house where only weak waves are received, it is recommended to receive data for a long time to detect such weak desired signals. Actually, however, it has been difficult to receive data for such a long time, since a phase rotation occurs in the pilot signal mainly due to the fading specific to wireless lines and/or a frequency shift to occur between the terminal oscillator and each base station oscillator. Hereunder, this conventional technique will be described with reference to FIG. 13. FIG. 13 shows incoherent combination, which is an object of the subject conventional technique. The incoherent combination means an operation for summing up power profiles converted from delay profiles obtained at different timings. The first step in FIG. 13 shows a received wave A and another received wave B delayed from the wave A. A position of a terminal is detected by measuring the receiving timing of the received signal that has a strong power and might be a direct wave (wave came through the line of-sight) or the preceding wave A that is or might be a direct wave. The second step in FIG. 13 shows a relationship between a time flow and information obtaining times. The horizontal axis in FIG. 13 denotes the time flow. For example, the data obtaining time differs between a signal received a T01 and a signal received at T02. Because each of the data obtaining times is an instantaneous one, each obtained information is referred to as a snapshot. The third step in FIG. 13 shows delay profiles generated from an obtained snapshot by the correlator 6 shown in FIG. 12. Although the output from the correlator 6 is a complex amplitude, it is denoted as incoherent-combined delay profiles converted from delay profiles so as to simplify the description.
In FIG. 13, signal waves A and B received at T01, T02, and T03 are affected by different fading levels or frequency shifts to occur between terminal and base station oscillators. The delay profiles in the third step in FIG. 13 thus change their shapes with time. After those delay profiles are combined incoherently, the fourth step shown in FIG. 13 is obtained. If delay profiles having a complex amplitude respectively are simply summed up before such the incoherent combination, their signal strengths will be cancelled by each other, since their signal vectors do not match, thereby their signals become weaker sometimes. Thus, receiving weak signals, which is an original object of the present invention, comes to be disabled.
The conventional problems described above will thus be summarized as follows; signals received in a long range cannot be used for coherent summation, because the complex amplitude of each delay profile changes mainly due to the fading and/or frequency shift to occur between terminal and base station oscillators.